Make sure QLoggingCategory default is available

This avoids crashes in case where qWarning() would be called from a
global static deleted after the one holding the default category. I
encountered this case with a QLockFile locked from a global static while
the application quits, it was calling qAppName which was triggering a
qWarning as the qapplication object was gone too.

Change-Id: I8910e8559d063e8f0a737bae3da5edc481ab84d3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>